Migrate to new test server

[florianm]

Feature

GetODK have generously sponsored ruODK with a new test server ruodk.getodk.cloud. This replaces the ODK Central server of my previous employer, to which I will lose access in the near future.

This issue is to track the migration of test forms to the new server.
I will also remove the "sandbox account request", as test accounts will be maintained strictly within the limits of package maintenance and development.